Micturition Chapter 26: Anatomy of Urinary Bladder and Urethra

Anatomy of Urinary Bladder and Urethra

• The detrusor muscle is a smooth muscle in the bladder that extends in all directions, increasing pressure in the bladder to 40-60 mm Hg when contracted.
• The trigone is a small triangular area on the posterior wall of the bladder where the two ureters enter the bladder.
• The internal sphincter is a smooth muscle that normally keeps the bladder neck and posterior urethra empty of urine, preventing bladder emptying until pressure rises above a critical threshold.
• The external sphincter is a skeletal muscle under voluntary control, located in the urogenital diaphragm, and can consciously prevent urination.

Innervation of the Ureter and Urinary Bladder

• The ureter is innervated by both parasympathetic and sympathetic nervous systems, enhancing and inhibiting peristaltic contractions, respectively.
• The urinary bladder is innervated by:
• Autonomic nervous system:
  • Parasympathetic fibers (S2 and S3) detect bladder wall stretch and stimulate detrusor muscle contraction.
  • Sympathetic fibers (L2) mainly stimulate blood vessels and have little effect on bladder contraction.
• Somatic nervous system:
  • Skeletal motor fibers through pudendal nerve control the external sphincter.

Vesicoureteral Reflux and Micturition Reflex

• Vesicoureteral reflux occurs when bladder contraction during micturition does not completely occlude the ureter, leading to urine reflux into the ureter.
• This can cause ureter enlargement, increased pressure in the renal calyces, and kidney damage.
• The micturition reflex is an autonomic spinal cord reflex that empties the bladder when the tension in its walls rises above a threshold level.

Pain Sensation in the Ureters and Ureterorenal Reflex

• Blockage of a ureter (e.g., by a stone) causes intense reflex constriction, severe pain, and sympathetic reflex back to the kidney, constricting renal arterioles and decreasing urine output.

Voluntary Urination

• Voluntary urination involves contraction of abdominal muscles, increasing pressure in the bladder.
• Sensory signals from bladder stretch receptors are conducted to sacral segments, reflexively causing contraction of the bladder and simultaneous inhibition of the external urethral sphincter.